1. Field of the Invention
This invention relates to apparatus for collecting and concentrating solar radiation for the generation of electrical power. In particular, the invention particularly concerns the mounting of a solar reflector dish. In addition, features of the construction of a solar radiation receiver and of a parabolic reflector dish are also described.
2. Description of the Prior Art
Systems are known for the generation of electrical power through the conversion of thermal energy produced by the concentration of solar energy by a suitable reflector, such as parabolic trough systems, planar mirror array systems and parabolic reflector dish systems.
In a parabolic reflector dish system, one or more parabolic dishes, each having a reflective surface, are driven in azimuth and elevation so as to track the diurnal and seasonal movement of the sun in order to collect and concentrate solar radiation in or on a suitable receiver. At the receiver, the thermal energy produced by the concentration of solar radiation is usually conducted away from the receiver to a heat engine, generator or the like for the production of electrical power.
In order for a parabolic reflector dish system to be of greatest effect, the area of the parabolic reflector dishes must be as large as possible. This is achieved either by having a large number of small reflector dishes or by having one, or only a small number of, very large reflector dish.
In systems comprising many reflector dishes, the cost of providing an equivalent number of drive systems so that each dish accurately tracks the movement of the sun becomes prohibitively expensive. Accordingly, large parabolic reflector dishes are frequently employed.
Unfocussed solar radiation has a peak solar flux of 1000 W/m.sup.2 at ground level. At a latitude of between 30.degree. and 35.degree., the average power of unfocussed solar radiation during each day is approximately 700 W/m.sup.2. At a typical system efficiency of about 30% therefore, in order to produce 1 Mw of electrical power, a parabolic reflector dish of about 4-5000 m.sup.2, or of about 75 m diameter would be required.
In order to mount and to move such a dish so as to track the sun, a large dish support structure is required. However, a large dish presents a considerable cross-section to any wind, and so as to remain accurately aligned to the sun, the dish support structure must be made sufficiently strong to resist wind forces. In very high winds, both the dish and the support structure are liable to be damaged, thus both need to be strengthened to resist the effect of occasional high winds. Reinforcing the dish and the support structure increases the mass of the dish which leads to an increase in their cost and also in the costs of the systems which move the dish so as to track the sun.
It is also usual for parabolic dish reflectors to be gimbally mounted, i.e. so as to pivot about an axis lying in a plane at the center of the dish. Such a mounting enables the dish to be moved in both azimuth and elevation using simple drive means, and the mass of the dish can be counterbalanced. However, the gimbal mounting of a dish means that the dish must be supported so as to pivot about an axis which is at a height above the ground equal to the radius of the dish. In the event of high winds, such a dish cannot be moved out of the path of the wind, and therefore the dish support structure has to be reinforced adding to the mass and cost thereof.
Due to such factors, current parabolic solar radiation reflector dishes are limited in size to between 10 m and 15 m in diameter. Accordingly, several such dishes are required in order to produce a significant amount of power (25 dishes of 15 m diameter being required to produce the same power as a single 75 m diameter dish), which becomes prohibitively expensive, as explained above.
FR-A-2535033 discloses an orientable solar colelctor. In the survival position, the dish of the orientable solar collector is located at a horizontal level approximately equal to or lower than that occupied by its lowest part for any operating position. Means are provided at the place of implantation of the collector to protect the structure and the supporting device from the wind. The latter comprises elevating and orienting means connecting the structure to a baseplate and being actuated by at least one driving member in order to control the orientation of the surface at least in site and the elevation thereof relative to said horizontal level. Means are also provided to compare the measured speed of the wind with a predetermined maximum permissible speed which is variable according to the site, in order, if applicable, to impose on the structure an orientation according to a site which is greater than or equal to a minimum value compatible with the measured speed of the wind.
In order for apparatus as disclosed in FR-A-2 535 033 to avoid obstruction of radiation to a portion of the dish, means must be provided to lift the dish up so that it is clear from any shadow created by the excavated hole or bark of earth.